Chance in Darwin's time was regarded as the product of human ignorance, that is to say chance is merely epistemic, not real like the ontological randomness of today's quantum indeterminacy.

Real chance was regarded as atheistic by most physical scientists since Newton, by philosophers like David Hume, and especially by the great mathematicians who developed the theories of probability and statistics that made chance a quantitative concept.

Darwin's use of the word "chance" in The Origin of Species is overwhelmingly to describe the chances of acquiring new characters and the chances of survival, and only rarely to the role of chance in the genetic variations that drive natural selection. He is reluctant to describe the details of genetic variation, perhaps because ascribing it simply to chance is scientifically unsatisfying. When he does come to connect chance to variation, he takes chance to be the result of human ignorance, leaving the door open to a better explanation in the future?

I HAVE hitherto sometimes spoken as if the variations so common and multiform in organic beings under domestication, and in a lesser degree in those in a state of nature had been due to chance. This, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation. Some authors believe it to be as much the function of the reproductive system to produce individual differences, or very slight deviations of structure, as to make the child like its parents. But the much greater variability, as well as the greater frequency of monstrosities, under domestication or cultivation, than under nature, leads me to believe that deviations of structure are in some way due to the nature of the conditions of life, to which the parents and their more remote ancestors have been exposed during several generations.
(The Origin of Species, chapter V, Laws of Variation.)

Darwin's Notebooks, especially his "transmutation notebooks," and the later M and N metaphysical notebooks, record some of Darwin's brief musings on the connection between "free will" and chance.

In his Notebooks on Man, Mind and Materialism, Darwin says...

Now it is not a little remarkable that the fixed laws of nature should be /universally/ thought to be the will of a superior being, whose nature can only be rudely traced out. When one sees this, one suspects that our will may /arise from/ as fixed laws of organization. M. le Comte argues against all contrivance — it is what my views tend to.

Darwin thinks the fixed laws of nature are enough to explain the world in materialistic terms. No superior contriving being is required. Note the similarity to Democritus.

With respect to free will, seeing a puppy playing cannot doubt that they have free will, if so all animals, then an oyster has & a polype (& a plant in some senses, perhaps, though from not having pain or pleasure, actions unavoidable & only to be changed by habits). Now free will of oyster, one can fancy to be direct effect of organization, by the capacities its senses give it of pain or pleasure. If so free will is to mind, what chance is to matter /M. Le Compte/

Darwin connects free will with chance, but it is epistemic chance. It produces random new possibilities, but they are completely determined by natural laws.

the free will (if so called) makes change in bodily organization of oyster, so may free will make change in man. — the real argument fixes on hereditary disposition & instincts. — Put it so. — Probably some error in argument, should be grateful if it were pointed out. My wish to improve my temper, what does it arise from, but organization, that organization may have been affected by circumstances & education & by the choice which at that time organization gave me to will — Verily the faults of the fathers, corporeal & bodily, are visited upon the children.—

Darwin sees a multiplicity of causes - hereditary, education, circumstances - that appear random and may not be known exactly. They show up as new organization (information structures?) in the individual.

The above views would make a man a predestinarian of a new kind, because he would tend to be an atheist. Man thus believing, would more earnestly pray "deliver us from temptation," he would be most humble, he would strive /to improve his organization/ for his children's sake & for the effect of his example on others. It may be doubted whether a man intentionally can wag his finger from real caprice. it is chance which way it will be, but yet it is settled by reason.

Darwin's new kind of predestination is purely material - the result of physical laws - and not the "contrivances" or "designs" of a deity. He appears to combine chance and reason in a two-stage view - "chance it will be, yet settled by reason" - that sounds like evolution?

what they teach by the same means & therefore properly no free will. — we may easily fancy there is, as we fancy there is such a thing as chance. — chance governs the descent of a farthing, free will determines our throwing it up, — equally true the two statements.

One is tempted to believe phrenologists are right about habitual exercise of the mind, altering form of head, & thus these qualities become hereditary. When a man says I will improve my powers of imagination, & does so, is not this free will, — he improves the faculty according to usual method, but what urges him, — absolute free will, motive may be anything ambition, avarice, etc., etc. An animal improves because its appetites urge it to certain actions, which are modified by circumstances, & thus the appetites themselves become changed. — appetites urge the man, but indefinitely, he chooses (but what makes him fix!? frame of mind, though perhaps he chooses wrongly, —& what is frame of mind owing to)

I verily believe free will & chance are synonymous. — Shake ten thousand grains of sand together & one will be uppermost, — so in thoughts, one will rise according to law.

Darwin drew a bracket of emphasis alongside the sentence above in his notebook. The preliminary random shaking stage, followed by a lawful rise, strongly suggests the Cogito two-stage model. The etymology of cogito is to shake together (co-agitare).

Darwin's opinions on free will are important, because his two-step process of chance-driven genetic variation followed by lawful natural selection is so similar to modern two-stage models of free will, including the first such model, proposed by William James.

James was directly inspired by Darwin's biological evolution in his 1880 model of "mental evolution."

"A remarkable parallel, which I think has never been noticed, obtains between the facts of social evolution on the one hand, and of zoölogical evolution as expounded by Mr. Darwin on the other..."

"[In mental evolution], if anywhere, it would seem at first sight as if that school must be right which makes the mind passively plastic, and the environment actively productive of the form and order of its conceptions...It might, accordingly, seem as if there were no room for any agency other than this…as if, in a word, the parallel with Darwinism might no longer obtain...

But, in spite of all these facts, I have no hesitation whatever in holding firm to the Darwinian distinction even here. I maintain that the facts in question are all drawn from the lower strata of the mind, so to speak.

And I can easily show...that as a matter of fact the new conceptions, emotions, and active tendencies which evolve are originally produced in the shape of random images, fancies, accidental out-births of spontaneous variation in the functional activity of the excessively instable human brain."
(James, William. 1880. "Great Men, Great Thoughts, and the Environment," Atlantic Monthly, vol.46, p.441-445)

Because James thinks "absolute chance" is real, the Darwinian scholar Robert J. Richards thinks Darwin himself would not have approved of James’s use of his evolutionary theory to defend free will. Richards says Darwin "was fully persuaded that human mental behavior was completely determined." He points out that James could not have known Darwin’s deterministic views, since they only appear in his notebooks, which had not been published when James wrote.
(Robert J. Richards, 1989, Darwin and the emergence of evolutionary theories of mind and behavior. U. Chicago Press, p.428n)

But Darwin might have liked Daniel Dennett's two-stage model of free will. Like Darwin, Dennett thinks random chance in the first stage is merely "pseudo-randomness," the kind generated by a deterministic computer or the randomness of deterministic chaos. Quantum indeterminacy is not required, Dennett says.

The members of society are deemed to have free will - and Darwin
ponders how the conception of free will could have evolved. The answer
he gives in the M notebook, page 72, is that "free will is to mind, what
chance is to matter," with chance and matter understood in the
Laplacian sense. The question therefore becomes: Given that at the
phenomenological level man has free will, how does justice get established
in the moral sphere and how is the stability of the social order
maintained? The premise of Scottish economic thought from Adam
Smith onward is that all the actors on the economic scene are free
agents. Yet, there are economic laws, and it is possible to predict the
consequences of certain events, such as a change in prices or a cut in
wages. How does this come about? I believe there is a parallel between
the role freedom of choice and free will play in Scottish economic and
moral science and the role played by random variations in the theory of
evolution. Both are the "chance" elements which are coupled to strongly
constraining (antichance) elements to give rise to a "lawful" theory.
("The Origin of the Origin Revisited," Journal of the History of Biology, Vol. 10, No. 2, p.233)

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The Notebooks on Man, Mind and Materialism

Aug. 12th. 38. At the Athenaeum Club was very much struck with intense an headache /after good days work/ which came on from reading (review of) M. Comte Phil. which made me /endeavour to/ remember, & to think deeply, & the immediate manner in which my head got well when reading article by Boz.

Perhaps one cause of the intense labour of original inventive thought is that none of the ideas are habitual, nor recalled by obvious associations. as by reading a book.—Consider this— |

August 29th. Went to Bed & built /common/ Castle in the air, of being compelled, from some quite imaginary cause to start at once to Shrewsbury, vaguely thought of packing up.—was lying on my back fell to sleep for second & wakened.—had very clear & pretty vivid /& perfectly characterized/ dream, in continuation of waking thought—my servant was in the room, with my trunk out & I was engaged in hurriedly giving orders.—Now what was difference between Castle & dream | No answer shows our profound ignorance in so simple case.—There was memory, for it related to past idea.—there was a kind of ideot consciousness for moment, implied by /presence/ my servant, /box,/ my own manner of ordering things to be done.—The senses are closed probably by sleep & not vice versa. anyhow I might have been quite still, & not attending to bodily sensations & yet the Castle would not have turned into dream.—It appears to me, that the mind is wholly absorbed with one idea (hence apparent vividness) & there being no other parallel trains of ideas connected with past circumstances, as whether I really was going to Shrewsbury, whether I had rung for Covington, whether he had come & opened box, whether I had thought what clothes to take (how often | one cannot tell whether one has rung the bell, when one recollects circumstances were such one naturally would do so!) Now all these parallel trains of thought necessary heirs of every action, & always running on in mind, being absent. one could not compare the castle with them. therefore could not doubt or believe. When I say trains, it may be instantaneous changes in order calling up ideas of every late impression.—(do the ideas, direct effect of perception by senses fail first, as whether I had pulled the bell??).—It may be deception to say the mind quicker in sleep, it may do less work & yet do so, from the exertion of keeping up the memory of every late impression. & likewise gaining new ones from senses. & calling up old ones, to be sure of one's consciousness. |

Mayo observes no improbabilities in a dream, effect of doubting nor believing, effect of not reasoning, effect of not having other trains of thought, or memory from innumerable late events.

September 3d Why when one thinks of any object (or having looked at any object one shuts ones eyes) is the image not vivid as in sleep — (one can dream of intense scarlet??) is it because one then has no immediate comparison with perceptions, & that one fancies the image more vivid? — Surely the image in a dream cannot truly be as vivid /as reality [readily?]/ as in Spectral images.—